1. Field of the Invention
This invention relates to a coating device which can uniformize the thickness and the composition of a coating to be formed by providing a member for uniformizing the flow of a paint gushing out into a coating bath and also to a coating method by use of said coating device.
2. Description of the Prior Art
Dip coating, which comprises dipping an object to be coated in a coating bath containing a paint and withdrawing the dipped object, is a coating method which has widely been employed. In this method, a coating bath of any desired shape may be used. A paints is charged in a coating bath, and in order to prevent nonuniformity of concentration of a paint or to prevent drying of the liquid surface of the paint, it is more preferable to gush out the paint upward from the inlet at the lower point of a coating bath to permit the paint to overflow from the bath rather than letting the paint stay in the bath, as disclosed in U.S. Pat. Nos. 2,808,344 and 3,046,156. Also, in this case, unevenness of coating is reduced by feeding constantly a paint to a coating bath to overflow continuously rather than by permitting it to overflow only at the time when a coating object is dipped in a paint.
In the method for feeding continuously a paint to a coating bath, it is reasonable to collect the overflowing paint and return it to the coating bath. As an improvement of this method, for the purpose of preventing fluctuations of the liquid surface in the coating tank, making the paint preparation easy and further retaining uniformity of the paint, it is proposed to provide additionally another tank separated from the coating bath and circulate the paint therebetween, as disclosed in Japanese patent application No. 200909/1982. The method is shown in FIG. 1. More specifically, a paint 7 is pressurized by means of a pump 8, filtered through a filter 9 and fed through a feeding inlet 10 into a coating bath 2, and the paint 7 is gushed out upward from the feeding inlet at the lower part of the coating bath to overflow and is collected in a receiving tray 4 before it is returned through a pipe 5 to the coating tank.
According to this method, the paint fed into the coating bath is desired to form a uniform and smooth flow around a material to be coated in the coating bath 2. In other words, the paint fed into the coating bath, which flows through a feeding inlet into the bath at a certain flow velocity, may agitate the paint in the bath or ripple the liquid surface of the paint when flowing linearly through the feeding inlet into the coating bath, whereby unevenness of the coating may be caused to occur. For example, when uniformity of film thickness contributes largely to electrophotographic characteristics as in the case of a photosensitive layer of an electrophotographic photosensitive member, it is supremely important to remove the unevenness.
Also, in the case when the surface of an object of which one end portion is opened and the other end portion is closed, is coated with a paint, and the object to be coated is dipped in the paint, a slight temperature difference between the paint and the ambience may frequently result in expansion of the air enclosed in the object or increase of gas volume by evaporation of the solvent of the coating liquid into the hollow of the coating object, thereby causing formation of a bubble from the lower end in the course of withdrawal of the object. The coating liquid will be moved greatly by generation of such a bubble to form unevenness on the coated surface, thus causing inconvenience in formation of a uniform coating.
In order to prevent such a trouble, it is proposed to remove a part of the air enclosed in the coating object in the coating step, as disclosed in Japanese patent application No. 173558/1981 (Japanese Laid-open Publication No. 74170/1983). This method is shown in FIG. 2, and the principle comprises removing only a part of the air enclosed in the object to be coated by means of an air pipe 14. More specifically, a screw 17 rotates by means of a motor 18 and the object to be coated held by a holding member 16 is drawn up. In the step of drawing up, for the purpose of preventing the bubble from inside the hollow from being released into a paint 13 contained in a coating bath 12 by rise of the pressure in the hollow of the object to be coated, a part of the air in the hollow of the object to be coated is permitted to be removed through an air pipe 14 connected to an air chamber made of a rubber.